Copper nanorod-based thermal interface material (TIM)

ABSTRACT

A copper nanorod thermal interface material (TIM) is described. The copper nanorod TIM includes a plurality of copper nanorods having a first end thermally coupled with a first surface, and a second end extending toward a second surface. A plurality of copper nanorod branches are formed on the second end. The copper nanorod branches are metallurgically bonded to a second surface. The first surface may be the back side of a die. The second surface may be a heat spread or a second die. The TIM may include a matrix material surrounding the copper nanorods. In an embodiment, the copper nanorods are formed in clusters.

BACKGROUND

Conventional semiconductor package structures include a microprocessor,or die, mounted to a package substrate. Typically, an integral heatspreader (IHS) overlies the backside of the die. A thermal interfacematerial (TIM) transfers heat generated by the die to the IHS, whichthen conducts the heat away from the die to additional heat removalelements, such as heat sinks. Die power consumption, die size, and heatdensity increases with each new generation of microprocessors, requiringhigher performance heat removal solutions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a cross-sectional side view of a semiconductorpackage having a die thermally coupled with a heat spreader by a thermalinterface material including copper nanorods, according to an embodimentof the invention.

FIG. 1B illustrates a cross-sectional side view of a thermal interfacematerial including clusters of copper nanorods thermally coupling thebackside of a die to a heat spreader, according to an embodiment of theinvention.

FIG. 1C illustrates a top-down view of a thermal interface materialincluding clusters of copper nanorods disposed on the backside of a die,according to an embodiment of the invention.

FIG. 2 illustrates a cross-sectional side view of a thermal interfacematerial including clusters of copper nanorods thermally coupling athermally warped backside of a die to a thermally warped heat spreader,according to an embodiment of the invention.

FIG. 3 illustrates a cross-sectional side view of a thermal interfacematerial including clusters of copper nanorods thermally coupling thebackside of a first die to the front side of a second die, according toan embodiment of the invention.

FIG. 4 illustrates a cross-sectional side view of a stacked diestructure having a thermal interface material including clusters ofcopper nanorods thermally coupling the backside of a first die to theactive side of a second die, and also thermally coupling the backside ofa second die to a heat spreader, according to an embodiment of theinvention.

FIG. 5 illustrates a cross-sectional side view of a thermal interfacematerial including copper nanorods thermally coupling the backside of adie to a heat spreader, according to an embodiment of the invention.

FIGS. 6A-6E illustrates cross-sectional side views of a method forforming a thermal interface material including copper nanorods thermallycoupling a die to a heat spreader, according to an embodiment of theinvention.

FIGS. 7A-7C illustrate three-dimensional perspective views of a coppernanorod with copper nanorod branches, grown from a surface according toan embodiment of the invention.

FIG. 8 illustrates a computing system implemented with a thermalinterface material including copper nanorods, in accordance with anexample embodiment of the invention.

DETAILED DESCRIPTION

A thermal interface material (TIM) having copper nanorods and a methodof forming the TIM to thermally couple two surfaces are described. Invarious embodiments, description is made with reference to figures.However, certain embodiments may be practiced without one or more ofthese specific details, or in combination with other known methods andconfigurations. In the following description, numerous specific detailsare set forth, such as specific configurations, dimensions andprocesses, etc., in order to provide a thorough understanding of thepresent invention. In other instances, well-known semiconductorprocesses and manufacturing techniques have not been described inparticular detail in order to not unnecessarily obscure the presentinvention. Reference throughout this specification to “one embodiment,”“an embodiment” or the like means that a particular feature, structure,configuration, or characteristic described in connection with theembodiment is included in at least one embodiment of the invention.Thus, the appearances of the phrase “in one embodiment,” “an embodiment”or the like in various places throughout this specification are notnecessarily referring to the same embodiment of the invention.Furthermore, the particular features, structures, configurations, orcharacteristics may be combined in any suitable manner in one or moreembodiment.

The terms “over”, “to”, “between” and “on” as used herein may refer to arelative position of one layer with respect to other layers. One layer“over” or “on” another layer or bonded “to” another layer may bedirectly in contact with the other layer or may have one or moreintervening layers. One layer “between” layers may be directly incontact with the layers or may have one or more intervening layers.

Conventional thermal interface materials (TIMs) include solders, epoxyresins filled with metal or ceramic particles, thermal greases, andphase change materials. Such TIMs typically have a thermal resistancegreater than 0.06 C-cm²/W and a bond line thickness greater than 20 μmfor polymer TIM and greater than 200 um for solder TIM. High thermalstresses evolve at the thermal interface due to the differentcoefficients of thermal expansion (CTE) of the integral heat spreader(IHS) and the die. Solders and epoxy resins have a high modulus, in theGPa range, which can lead to delamination when stresses evolve at theTIM/IHS interface or the TIM/die interface. Delamination results inincreased thermal resistance due to reduction of the thermal path. Whilethermal greases and phase change materials have a lower modulus, thermalgreases may be pumped out of the thermal interface during thermalcycling and phase change materials have poor thermal conductivity.

In one aspect, embodiments of the invention describe a thermal interfacematerial having ultra-low thermal resistance, thin form factor, lighterweight, and lower cost. Copper nanorods extend from the backside of thedie to the IHS. The copper nanorods are metallurgically bonded to theIHS, providing low thermal resistance at the TIM/IHS interface. Inaddition, the copper nanorods may be metallurgically bonded to thebackside of the die, providing a low contact resistance at the TIM/dieinterface. The copper nanorods have high bulk thermal conductivity,greater than 100 W/m/K. In addition, the copper nanorods have a lengthfrom 20 to 50 μm, enabling a bond line thickness (BLT) less than 50 μm.The combination of a low interfacial thermal resistance, high bulkthermal conductivity, and low BLT enables an ultra-low thermalresistance of less than 0.02 C-cm²/W. The low BLT also enables areduction in overall package z-height, for example, compared to packagesin which solder TIM is used, which helps to achieve a lower form factoruseful for mobile, tablet, and laptop applications. And further, the lowBLT enables use of less material, resulting in a lighter overallpackage. Less material also contributes to a lower cost as compared tosome conventional TIMs, such as solders.

In another aspect, the thermal interface material is able to accommodateCTE mismatch between the die and the IHS due to the high aspect ratioand corresponding flexibility of the copper nanorods. For example, thedie and/or the IHS may warp due to thermal stresses, which may lead todifferences in the BLT near edges of the die as compared to the BLT nearthe center of the die. These changes in BLT may place portions of theTIM under compressive or tensile stress. The copper nanorods may flexand/or deform to accommodate stresses without delaminating from the dieor IHS.

In yet another aspect, the thermal interface material may be formed atlow temperatures. For example, the copper nanorods and branches may begrown at room temperature. Then, the copper nanorod branches may bemetallurgically bonded to the IHS at a temperature below 200° C. Lowtemperature processing reduces the risk for failure due to thermalstresses and also reduces the internal stress of the TIM.

In yet another aspect, the copper nanorod TIM may be formed at theindividual device level, at the wafer level, or formed on a sacrificialsurface and then applied to the thermal interface. For example, thecopper nanorods may be grown over the back surface of an individual dieor over the surface of an IHS. In another embodiment, the copper nanorodTIM may be formed over the surface of a wafer including multiple diesthat have not yet been singulated. In yet another embodiment, the coppernanorod TIM may be formed on a sacrificial film or surface, and thentransferred to a wafer or a die.

FIG. 1A illustrates a cross-sectional side view of a package 100 havinga die 102 thermally coupled with an integral heat spreader (IHS) 110 bycopper nanorod thermal interface material (TIM) 114, according to anembodiment of the invention. In an embodiment, die 102 is mounted tosubstrate 104 by bumps 106. The space between die 102 and substrate 104may be underfilled with underfill material 108. Substrate 104 may haveinterconnects 116 formed on contact pads 118 on the surface opposite die102. Interconnects 116 may be used, for example, to mount package 100 toa printed circuit board (not shown).

In an embodiment, IHS 110 overlies die 102. In an embodiment, IHS 110dissipates heat generated by die 102 by conducting heat away from die102 toward the periphery of the package 100. IHS 110 may also conductheat away from die 102 to additional heat transfer components. In anembodiment, IHS 110 encloses die 102. In an embodiment, IHS 110 iscoupled with substrate 104 by adhesive 112. IHS 110 may overlie orenclose additional dies and/or package components (not shown). IHS 110is formed from a material that conducts heat, for example, a metal. Inan embodiment, IHS 110 is copper. IHS 110 may also be, for example,aluminum.

FIG. 1B is a close-up view of section 130 of FIG. 1A, showing coppernanorod TIM 114 thermally joining IHS 110 and the backside of die 102.In an embodiment, copper nanorod TIM 114 includes copper nanorods 120.In an embodiment, copper nanorod TIM 114 includes seed layer 124, formedover the backside surface of die 102. Copper nanorods 120 have a firstend 117 thermally coupled with seed layer 124 and a second end 119extending toward overlying IHS 110, according to an embodiment. Inanother embodiment, copper nanorods 120 are grown directly from the backsurface of die 102. In an embodiment, one or more copper nanorods 120are grouped in to clusters 121. In an embodiment, copper nanorods 120have copper nanorod branches 122 extending from second end 119. In anembodiment, copper nanorod branches 122 are metallurgically bonded toIHS 110.

Copper nanorods 120 conduct heat between the backside surface of die 102to IHS 110, according to an embodiment. In an embodiment, coppernanorods 120 are a single crystal of copper. In an embodiment, the highbulk thermal conductivity (˜400 W/m/K) of copper enables effectivethermal conduction by copper nanorods 120 through TIM 114. The length ofcopper nanorods 120 enables a low BLT, for example, less than 100 μm,according to an embodiment. In an embodiment, the BLT is less than 50μm. In another embodiment, the BLT is from 20-30 μm. In an embodiment,copper nanorods 120 have a length from 20-50 μm. In an embodiment,copper nanorods have a length from 25-30 μm. In an embodiment, coppernanorods 120 have a diameter greater than 10 μm. In an embodiment,copper nanorods 120 have an aspect ratio from 1:2 to 1:10. In anembodiment, copper nanorods 120 have an average spacing greater than 10μm. In another embodiment, copper nanorods 120 have an average spacingfrom 50 to 100 μm.

Copper nanorod TIM 114 includes copper nanorod branches 122 formed oneach second end 119 of copper nanorods 120, according to an embodimentof the invention. In an embodiment, two copper nanorod branches areformed on each second end 119 of a copper nanorod 120. In an embodiment,copper nanorod branches 122 have a length less than 10 μm. In anembodiment, copper nanorod branches 122 have a length from 1 to 2 μm. Inan embodiment, copper nanorod branches 122 have a diameter that is smallenough to enable metallurgic bonding to a copper surface, such as thatof IHS 110. In an embodiment, copper nanorod branches 122 have adiameter from 10-100 nm. The metallurgical bonding of copper nanorodbranches 122 to IHS 110 enables a lower contact thermal resistance atthe interface of TIM 114 with IHS 110, contributing to an overall lowerthermal resistance for TIM 114.

In an embodiment, copper nanorod TIM 114 includes matrix material 126.Matrix material 126 may improve the overall conductivity of coppernanorod TIM 114 as compared to a copper nanorod TIM 114 without a matrixmaterial. In another embodiment, matrix material 126 improves themechanical rigidity of copper nanorod TIM 114, which in combination withthe ductility of the copper nanorods enables the TIM 114 to function asa membrane, allowing for a low BLT. In an embodiment, matrix material126 fills the spaces between IHS 110 and seed layer 124 that are notoccupied by copper nanorods 120. In an embodiment, matrix material 126fills the space between copper nanorod clusters 121. In an embodiment,matrix material 126 fills the space between copper nanorods 120 within acluster 121. In another embodiment, matrix material 126 does not fillthe space between copper nanorods 120 within a cluster 121. In anembodiment, the material composition of matrix material 126 is selectedto enable filling of the space between die 102 and IHS 110 surroundingcopper nanorods 120. Matrix material 126 may include conventionalunderfill materials, for example, but not limited to, epoxy. In anembodiment, matrix material 126 is filled with particles that improvethe thermal conductivity of matrix material 126 and TIM 114, forexample, but not limited to, metal or graphite fillers. In anembodiment, TIM 114 does not include a matrix material.

In an embodiment, the first ends 117 of copper nanorods 120 arethermally coupled to seed layer 124. In an embodiment, seed layer 124 isa material that facilitates the seeding and growth of copper nanorods120. Seed layer 124 may be selected to enable growth of copper nanorods120 having a desired crystallographic orientation. In an embodiment,seed layer 124 is formed over the entire backside surface of die 102. Inan embodiment, one or more adhesion layers and/or diffusion barriers(not shown) are located between seed layer 124 and the surface of die102. In an embodiment, seed layer 124 has been patterned to remove seedlayer 124 from areas where copper nanowires 120 are not desired, such asin order to define clusters 121. In yet another embodiment, coppernanorods 120 are grown directly on the backside surface of die 102. Inan embodiment, there is no seed layer 124. Seed layer 124 may be anymaterial from which copper nanorods may be grown. In an embodiment, seedlayer 124 is a material having a (110) crystallographic orientation. Inan embodiment, seed layer 124 is copper. In another embodiment, seedlayer 124 is silicon.

In an embodiment, copper nanorods 120 are grouped into clusters 121.Grouping copper nanorods 120 into clusters 121 may lower the modulus ofcopper nanorod TIM 114. In addition, grouping copper nanorods 120 intoclusters 121 may facilitate the inclusion of a matrix material 126 byproviding larger channels between clusters through which the matrixmaterial may flow during application. In an embodiment, clusters 121 mayinclude up to 25 copper nanorods 120. Within a cluster 121, coppernanorods 120 may have an average spacing greater than 10 μm. In anembodiment, the average spacing between copper nanorods 120 is from 10to 100 μm.

FIG. 1C illustrates a top-down view of TIM 114 over the backside surfaceof die 102 along line A-A′ in FIG. 1A, according to an embodiment of theinvention. Clusters 121 are shown on copper layer 124, according to anembodiment. The optional matrix material is not shown for clarity.Clusters 121 may have a variety of footprint shapes, such as thecircular shape shown in FIG. 1C. Clusters 121 may also have polygonal orirregular footprint shapes. The size and arrangement of clusters 121 mayvary based on a variety of factors, for example, but not limited to, thedesired properties of copper nanorod TIM 114 (e.g. the thermalresistance, the modulus, and cost), the dimensions of die 102, or byprocessing concerns, (e.g. the flow properties of matrix material 124).In an embodiment, clusters 121 have an average diameter D_(C). Averagediameter D_(C) may be from 10 to 50 μm. In an embodiment, clusters 121have an average spacing S_(C). In an embodiment, average spacing S_(C)is greater than 50 μm. In an embodiment, 10 to 50% of the surface areaof die 102 is occupied by copper nanorods 120. In an embodiment, 10 to50% of the volume between die 102 and IHS 110 is occupied by coppernanorods 120.

In another embodiment, the first end 117 of each copper nanorod 120 isthermally coupled to IHS 110, while second ends 119 extend toward die102. In an embodiment, copper nanorod branches 122 extend from secondend 119. In an embodiment, copper nanorod branches 122 may bemetallurgically bonded to a copper layer, such as layer 124, formed overthe backside of die 102.

FIG. 2 illustrates a cross-sectional view of a copper nanorod TIM 214thermally joining die 202 to IHS 210, according to an embodiment. TIM214 includes copper nanorods 220 grouped into clusters 221, according toan embodiment. In an embodiment, each copper nanorod 220 has a first end217 coupled with seed layer 224 formed over the surface of die 202, anda second end 219 extending toward IHS 210. In an embodiment, coppernanorod branches 222 are formed on second end 219. In an embodiment,matrix material 226 fills the space between copper nanorods 220.

Thermal stresses have warped die 202 and IHS 210, according to anembodiment. For example, the temperature of die 202 may change or cycleduring fabrication or operation. Thermal stresses may evolve due todifferences in the coefficient of thermal expansion (CTE) for thematerials included within die 202. Temperature changes cause differentmaterials within die 202 to expand and contract different amounts, whichmay cause warpage. Similarly, a temperature gradient within a copper IHS210 may cause warmer areas of IHS 210 to expand relative to coolerareas, which may lead to warpage.

Copper nanorod TIM 214 includes high aspect ratio copper nanorods 220,according to an embodiment. In an embodiment, the high aspect ratio ofcopper nanorods 220 allow TIM 214 to deform to accommodate the warpageof die 202 and/or IHS 210. In an embodiment, the ductility of the coppermay allow deformation to accommodate the warpage of die 202 and/or IHS210. By accommodating the warpage at the thermal interface, thermalcontact is preserved with both the die 202 surface and the IHS 210surface, preventing or minimizing delamination of the TIM 214 from thethermal interface, which can lead to increased thermal resistance andreduced heat management performance.

FIG. 3 illustrates a cross-sectional view of a copper nanorod TIM 314thermally joining first die 302 to second die 303, such as in apackage-on-package (PoP) or 3D package structure, according to anembodiment of the invention. In an embodiment, copper nanorod TIM 314includes copper nanorods 320 having a first end 317 thermally coupled toa seed layer 324 over the backside of first die 302 and a second end 319extending toward second die 303. Copper nanorods may be grouped intoclusters 321. A plurality of copper nanorod branches 322 extend fromeach second end 319, according to an embodiment. In an embodiment,copper nanorod branches 322 are metallurgically bonded to copper layer325, covering the surface of second die 303. In an embodiment, thelength of copper nanorods 310 within copper nanorod TIM 314 enables aBLT from 10 to 50 μm. In another embodiment, other device components,having a seed layer 324 or copper layer 325 formed thereon, may bethermally joined by a copper nanorod TIM 314.

FIG. 4 illustrates a cross-sectional view of a 3D package having acopper nanorod TIM 414A the interface first die 401 and second die 402and copper nanorod TIM 414B the interface of second die 402 and IHS 410,according to an embodiment of the invention. By incorporating a coppernanorod TIM 414A/414B at both thermal interfaces, heat is effectivelyconducted between dies in the die stack to the IHS 410 in order to beremoved. In an embodiment, copper nanorod TIM 414A includes coppernanorods 420 extending from a seed layer 424 over first die 402 towardIHS 410. Copper nanorod branches metallurgically bond the coppernanorods to the IHS 410, according to an embodiment. In an embodiment,copper nanorods are grouped in clusters 421. TIM 414A may include amatrix material 426.

In an embodiment, TIM 414B includes a seed layer 424 formed over thesurface of second die 401. Copper nanorods 420 have a first end 417coupled to seed layer 424 and a second end 419 extending toward firstdie 402, according to an embodiment. A copper layer 425 is formed overthe bottom surface of first die 402, according to an embodiment. Coppernanorod branches 422 formed on second end 419 are metallurgically bondedto copper layer 425, according to an embodiment.

FIG. 5 illustrates a cross-sectional view of copper nanorod TIM 514thermally joining a die 502 to an IHS 510, according to an embodiment ofthe invention. In an embodiment, copper nanorod TIM 514 includes coppernanorods 520 extending from the backside of die 502 toward IHS 510. Inan embodiment, copper nanorods 520 have a first end 517 coupled to seedlayer 524 over the surface of die 502, and a second end 519 extendingtowards IHS 510. Copper nanorods 520 may not be grouped into clusters,as shown in FIGS. 1-4. In an embodiment, copper nanorods 520 areuniformly distributed across the surface of seed layer 524. In anembodiment, copper nanorods 520 are randomly distributed across thesurface of seed layer 524. In an embodiment, copper nanorods 520 have anaverage spacing greater than 10 μm. In an embodiment, copper nanorods520 have an average spacing from 50 to 100 μm. In an embodiment, coppernanorod branches 522 metallurgically bond copper nanorods 520 to thesurface of IHS 510. In an embodiment, TIM 514 includes matrix material526 filling the space between copper nanorods 520.

FIGS. 6A-6E illustrate cross-sectional views of a method for forming acopper nanorod TIM, according to an embodiment of the invention. In FIG.6A, first surface 602 is provided. First surface 602 may be the surfaceof any heat-generating object or any heat-conduction object. Firstsurface may be, for example, but not limited to, a die or a heatspreader. In an embodiment, first surface 602 is the backside surface ofan individual die. In another embodiment, first surface 602 is thesurface of a wafer including a plurality of dies. In yet anotherembodiment, first surface 602 is a sacrificial or template surface, onwhich a copper nanorod TIM may be formed, then removed from the firstsurface 602 and transferred to a thermal interface.

In FIG. 6B, copper nanorods 620 are grown from the first surface 602,according to an embodiment of the invention. In an embodiment, coppernanorods 620 have a first end 617 coupled with first surface 602, and asecond end 619 extending away from first surface 602. Copper nanorods620 may be in clusters 621, as shown in FIGS. 1A-4 and discussed above,or unclustered, as show in FIG. 5 and discussed above. Copper nanorods620 may be grown from a variety of surfaces. For example, coppernanorods 620 may be grown directly from the silicon backside surface ofa die. In another embodiment, copper nanorods 620 are grown from a seedlayer 624 formed over first surface 602. Seed layer 624 may be anymaterial from which copper nanorods may be grown, for example a materialhaving an FCC crystallographic structure. Seed layer 624 may be, forexample, but not limited to, copper. In an embodiment, one or morelayers (not shown) are formed between first surface 602 and seed layer624 to improve adhesion and/or serve as a diffusion barrier.

In an embodiment, a patterning layer (not shown) is formed over the seedlayer 624. In an embodiment, the patterning layer self assembles todefine areas where copper nanorods may be grown. The patterning layermay be, for example, a monolayer of polystyrene spheres. In anotherembodiment, the patterning layer may be patterned to expose portions ofseed layer 624 where copper nanorods are desired, while coveringportions of seed layer 624 where copper nanorods are not desired. In anembodiment, a patterning layer is used to define copper nanorod clusters621. The patterning layer may then be removed after growth of coppernanorods 620. In another embodiment, a matrix material (for example,matrix material 626, as shown in FIG. 6E) may be used at the patterninglayer, in which case the matrix material may not be removed after growthof copper nanorods 620.

FIG. 7A illustrates a perspective view of an individual copper nanorod720, grown from seed layer 724, according to an embodiment of theinvention. In an embodiment, seed layer 724 is copper. In an embodiment,copper seed layer 724 has a {110} orientation in the direction normal tothe surface. Copper nanorod 720 is nucleated on the surface of seedlayer 724, and grown in the <110> direction, according to an embodiment.In an embodiment, copper nanorod 720 is a single crystal of copper.Copper nanorods 720 may be formed by sputtering or by glancing angledeposition (GLAD). By varying the orientation of the seed layer 724 andthe deposition angle, the orientation of the copper nanorod 720 and thedirection of growth may be altered. For example, copper nanorod 720 maybe grown in the <110> direction from the {110} surface of seed layer 724by sputtering or depositing at the appropriate incident angle. In anembodiment, an incident angle of 83-88° leads to growth from the {110}surface in the <110> direction. Deposition continues until the desiredlength, L_(N), is achieved. Copper nanorod 720 has a first end 717thermally coupled to seed layer 724, and a second end 719 protrudingaway from seed layer 724. Copper nanorod 720 may have a length L_(N)from first end 717 to second end 719 that is from 20 to 50 μm. In anembodiment, copper nanorods may be formed at room temperature. Inanother embodiment, copper nanorods may be formed at a temperature lessthan 100° C.

Referring now to FIG. 6C, copper nanorod branches 622 are grown from thesecond end 619 of copper nanorods 620, according to an embodiment of theinvention. FIGS. 7B-7C illustrate perspective views of copper nanorodbranches 722 grown from the second end 719 of copper nanorod 720,according to an embodiment. Due to the shadowing effect of GLAD, and theaccumulation of stacking faults on the top (110) face of the coppernanorod 720, copper atoms begin to assemble on the {111} facets of thenanorod, split apart by the stacking faults on the (110) face. Continuedgrowth forms two copper nanorod branches 722. Copper nanorod branches722 may have a length L_(B) less than 10 μm. In an embodiment, coppernanorod branches 722 have a length L_(B) from 1 to 2 μm. In anembodiment, copper nanorod branches 722 have a diameter from 10 to 100nm.

In an embodiment, copper nanorod branches 622 are then coated to inhibitthe formation of native oxide. Copper nanorod branches 622 may be coatedwith, for example, nickel. Any appropriate method may be used to coatcopper nanorod branches 622, such as, but not limited to, spray coat,PVD, and electrochemical coating. In an embodiment, copper nanorodbranches 622 are not coated.

Then, in FIG. 6D, second surface 610 is metallurgically bonded to coppernanorod branches 622, according to an embodiment of the invention. In anembodiment, second surface 610 is a copper IHS, as illustrated in FIG.6D. In another embodiment, second surface 610 is a die having a copperlayer formed thereon. In another embodiment, second surface 610 is asacrificial substrate having a copper layer formed thereon. Secondsurface 610 is placed in contact with copper nanorod branches 622 andheated to a temperature sufficient to metallurgically bond the coppersecond surface 610 and copper nanorod branches 622, according to anembodiment. Metallurgical bonding occurs at temperatures far below themelting temperature of the metal. For example, while copper melts atover 1000° C., copper rods and wires having a diameter less than 100 nmmay bond to a copper surface at temperatures less than 400° C. As such,by forming copper nanorod branches 622 on second ends 619 of coppernanorods 620, low-temperature metallurgical bonding may be used. Lowertemperature processing reduces the evolution of thermal stresses andassociated device failures. In an embodiment, copper nanorod branches622 are metallurgically bonded to second surface 610 at a temperaturefrom 200 to 400° C. In an embodiment, prior to metallurgically bondingcopper nanorod branches 622 to second surface 610, copper nanorodbranches 622 are treated with flux to remove oxide. In an embodiment,pressure is applied to thermal interface to assist in the formation of ametallurgic bond. Optionally, in FIG. 6E, matrix material 626 may beapplied to fill a portion of the remaining space between first surface602 and second surface 610. Matrix material 626 may be injected. In anembodiment, matrix material 626 is dispensed prior to assembly.

As such, a copper nanorod TIM 614 having reduced thermal resistance isformed. The high bulk conductivity of copper nanorods 620, low BLT, andlow contact resistance due to metallurgical bonding enable coppernanorod TIM 614 to have an ultra-low thermal resistance, according to anembodiment. In an embodiment, copper nanorod TIM 614 has a thermalresistance less that 0.3 C-cm²/W. In an embodiment, copper nanorod TIM614 has a thermal resistance from 0.1 to 0.2 C-cm²/W.

In an embodiment where copper nanorod TIM 614 was formed using asacrificial first surface 602 and a sacrificial second surface 610,copper nanorod TIM 614 is then removed from between first surface 602and second surface 610. Copper nanorod TIM 614 may then be applied to athermal interface, such as between a die and an IHS.

FIG. 8 illustrates a computing device 800 in accordance with oneimplementation of the invention. The computing device 800 houses a board802. The board 802 may include a number of components, including but notlimited to a processor 804 and at least one communication chip 806. Theprocessor 804 is physically and electrically coupled to the board 802.In some implementations the at least one communication chip 806 is alsophysically and electrically coupled to the board 802. In furtherimplementations, the communication chip 806 is part of the processor804.

Depending on its applications, computing device 800 may include othercomponents that may or may not be physically and electrically coupled tothe board 802. These other components include, but are not limited to,volatile memory (e.g., DRAM), non-volatile memory (e.g., ROM), flashmemory, a graphics processor, a digital signal processor, a cryptoprocessor, a chipset, an antenna, a display, a touchscreen display, atouchscreen controller, a battery, an audio codec, a video codec, apower amplifier, a global positioning system (GPS) device, a compass, anaccelerometer, a gyroscope, a speaker, a camera, and a mass storagedevice (such as hard disk drive, compact disk (CD), digital versatiledisk (DVD), and so forth).

The communication chip 806 enables wireless communications for thetransfer of data to and from the computing device 800. The term“wireless” and its derivatives may be used to describe circuits,devices, systems, methods, techniques, communications channels, etc.,that may communicate data through the use of modulated electromagneticradiation through a non-solid medium. The term does not imply that theassociated devices do not contain any wires, although in someembodiments they might not. The communication chip 806 may implement anyof a number of wireless standards or protocols, including but notlimited to Wi-Fi (IEEE 802.11 family), WiMAX (IEEE 802.16 family), IEEE802.20, long term evolution (LTE), Ev-DO, HSPA+, HSDPA+, HSUPA+, EDGE,GSM, GPRS, CDMA, TDMA, DECT, Bluetooth, derivatives thereof, as well asany other wireless protocols that are designated as 3G, 4G, 5G, andbeyond. The computing device 800 may include a plurality ofcommunication chips 806. For instance, a first communication chip 806may be dedicated to shorter range wireless communications such as Wi-Fiand Bluetooth and a second communication chip 806 may be dedicated tolonger range wireless communications such as GPS, EDGE, GPRS, CDMA,WiMAX, LTE, Ev-DO, and others. The communication chip 806 also includesan integrated circuit die packaged within the communication chip 806. Inan embodiment, communication chip 806 is thermally coupled to a thermalmanagement structure using a thermal interface material including coppernanorods metallurgically bonded to the thermal management structure, inaccordance with embodiments of the invention.

The processor 804 of the computing device 800 includes an integratedcircuit die packaged within the processor 804. In an embodiment,processor 804 is thermally coupled to a thermal management structureusing a thermal interface material including copper nanorodsmetallurgically bonded to the thermal management structure, inaccordance with embodiments of the invention. The term “processor” mayrefer to any device or portion of a device that processes electronicdata from registers and/or memory to transform that electronic data intoother electronic data that may be stored in registers and/or memory.

In various implementations, the computing device 800 may be a laptop, anetbook, a notebook, an ultrabook, a smartphone, a tablet, a personaldigital assistant (PDA), an ultra mobile PC, a mobile phone, a desktopcomputer, a server, a printer, a scanner, a monitor, a set-top box, anentertainment control unit, a digital camera, a portable music player,or a digital video recorder. In further implementations, the computingdevice 800 may be any other electronic device that processes data.

In an embodiment, a thermal interface material comprises a plurality ofcopper nanorods, each having a first end thermally coupled with a firstsurface and a second end extending toward a second surface, wherein thesecond surface is copper; and a plurality of copper nanorod branchesextending from each second end, wherein the copper nanorod branches aremetallurgically bonded to the second surface. In an embodiment, thethermal interface material has a thermal resistivity less than 0.03C-cm²/W. In an embodiment, the copper nanorods have a diameter less than20 μm. In an embodiment, the copper nanorods have a length from 5 μm to50 μm. In an embodiment, the copper nanorods are spaced from 5 nm to 10μm apart. In an embodiment, 10% to 50% of a surface area of the firstsurface is occupied by copper nanorods. In an embodiment, 10% to 50% ofa volume between the first surface and the second surface is occupied bycopper nanorods. In an embodiment, the copper nanorods are clustered ina plurality of nanorod clusters. In an embodiment, the nanorod clustershave an average diameter from 50 to 100 μm. In an embodiment, theclusters have an average spacing from 50 to 100 μm. In an embodiment,the thermal interface material further comprises a matrix materialbetween the first surface and the second surface. In an embodiment, thefirst surface is copper and the first ends are metallurgically bondedwith the first surface. In an embodiment, the copper nanorods have a<110> orientation along a central axis. In an embodiment, the copperbranches extend from <111> facets on the second end of the coppernanorods.

In an embodiment, a microelectronic package comprises a substrate; a dieelectrically coupled with the substrate; an integrated heat spreaderoverlying the die; and a thermal interface material thermally couplingthe integrated heat spreader with a back surface of the die, wherein theTIM comprises: a plurality of copper nanorods, each having a first endthermally coupled with the back surface of the die and a second endextending toward the integrated heat spreader; and a plurality of coppernanorod branches extending from each second end, wherein the coppernanorod branches are metallurgically bonded to the integrated heatspreader. In an embodiment, a bond line thickness (BLT) between the backsurface of the die and the integrated heat spreader is less than 50 μm.In an embodiment, the copper nanorods have a diameter less than 20 μm.In an embodiment, the thermal interface material further comprises amatrix material between the back surface of the first die and theintegrated heat spreader. In an embodiment, the thermal interfacematerial further comprises a copper layer formed over the back surfaceof the die, wherein the first ends are metallurgically bonded with thecopper layer. In an embodiment, the nanorods are clustered in aplurality of nanorod clusters.

In an embodiment, a microelectronic package comprises a first die havinga first surface; a second die having a second surface; and a thermalinterface material thermally coupling the first surface with the secondsurface, wherein the thermal interface material comprises: a copperlayer formed over the second surface; a plurality of copper nanorods,each having a first end thermally coupled with the first surface and asecond end extending toward the copper layer; and a plurality of coppernanorod branches extending from each second end, wherein the coppernanorod branches are metallurgically bonded to the copper layer. In anembodiment, the thermal interface material further comprises a matrixmaterial between the back surface of the first die and the copper layer.In an embodiment, the nanorods are clustered in a plurality of nanorodclusters.

In an embodiment, a method for thermally coupling a first surface and asecond surface comprises growing a plurality of copper nanorods from thefirst surface, wherein the copper nanorods have a first end thermallycoupled with the first surface and a second end extending away from thefirst surface; growing a plurality of copper nanorod branches on eachsecond end; and metallurgically bonding the second surface to the coppernanorod branches, wherein the second surface is copper. In anembodiment, the method further comprises, prior to growing a pluralityof copper nanorods, forming a patterning layer over the first surface;and patterning the patterning layer to expose portions of the firstsurface. In an embodiment, metallurgically bonding the second surface tothe copper nanorod branches comprises heating the nanorod branches andthe second surface to a temperature less than 400° C. In an embodiment,the method further comprises underfilling the remaining space betweenthe first surface and the second surface with a matrix material. In anembodiment, the method further comprises forming a nickel layer on thecopper nanorods and copper nanorod branches to prevent oxidation. In anembodiment, the method further comprises introducing a fluxing agent toremove oxide from the copper nanorods and copper nanorod branches. In anembodiment, the copper nanorods are grown by one of sputter depositionand vapor deposition.

Although the invention has been described with reference to specificembodiments, it will be understood by those skilled in the art thatvarious changes may be made without departing from the spirit or scopeof the invention. Accordingly, the disclosure of embodiments of theinvention is intended to be illustrative of the scope of the inventionand is not intended to be limiting. It is intended that the scope of theinvention shall be limited only to the extent required by the appendedclaims. For example, to one of ordinary skill in the art, it will bereadily apparent that the internal spacers and the related structuresand methods discussed herein may be implemented in a variety ofembodiments, and that the foregoing discussion of certain of theseembodiments does not necessarily represent a complete description of allpossible embodiments.

Additionally, benefits, other advantages, and solutions to problems havebeen described with regard to specific embodiments. The benefits,advantages, solutions to problems, and any element or elements that maycause any benefit, advantage, or solution to occur or become morepronounced, however, are not to be construed as critical, required, oressential features or elements of any or all of the claims.

Moreover, embodiments and limitations disclosed herein are not dedicatedto the public under the doctrine of dedication if the embodiments and/orlimitations: (1) are not expressly claimed in the claims; and (2) are orare potentially equivalents of express elements and/or limitations inthe claims under the doctrine of equivalents.

What is claimed is:
 1. A thermal interface material, comprising: aplurality of copper nanorods, each having a first end thermally coupledwith a first surface and a second end extending toward a second surface,wherein the second surface is copper; and a plurality of copper nanorodbranches extending from each second end, wherein the copper nanorodbranches have a Cu—Cu metallurgic bond to the second surface.
 2. Thethermal interface material of claim 1, wherein the thermal interfacematerial has a thermal resistivity less than 0.03 C-cm²/W.
 3. Thethermal interface material of claim 1, wherein the copper nanorods havea diameter less than 20 μm.
 4. The thermal interface material of claim1, wherein the copper nanorods have a length from 5 μm to 50 μm.
 5. Thethermal interface material of claim 1, wherein the copper nanorods arespaced from 5 nm to 10 μm apart.
 6. The thermal interface material ofclaim 1, wherein 10% to 50% of a surface area of the first surface isoccupied by copper nanorods.
 7. The thermal interface material of claim1, wherein 10% to 50% of a volume between the first surface and thesecond surface is occupied by copper nanorods.
 8. The thermal interfacematerial of claim 1, wherein the copper nanorods are clustered in aplurality of nanorod clusters.
 9. The thermal interface material ofclaim 8, wherein the nanorod clusters have an average diameter from 50to 100 μm.
 10. The thermal interface material of claim 8, wherein theclusters have an average spacing from 50 to 100 μm.
 11. The thermalinterface material of claim 1, further comprising a matrix materialbetween the first surface and the second surface.
 12. The thermalinterface material of claim 1, wherein the first surface is copper, andwherein the first ends are metallurgically bonded with the firstsurface.
 13. The thermal interface material of claim 1, wherein thecopper nanorods have a <110> orientation along a central axis.
 14. Thethermal interface material of claim 1, wherein the copper branchesextend from <111> facets on the second end of the copper nanorods.
 15. Amethod for thermally coupling a first surface and a second surface,comprising: growing a plurality of copper nanorods from the firstsurface, wherein the copper nanorods have a first end thermally coupledwith the first surface and a second end extending away from the firstsurface; growing a plurality of copper nanorod branches on each secondend; and metallurgically bonding the second surface to the coppernanorod branches, wherein the second surface is copper wherein thecopper nanorod branches have a Cu—Cu metallurgic bond to the secondsurface.
 16. The method of claim 15, further comprising, prior togrowing a plurality of copper nanorods: forming a patterning layer overthe first surface; and patterning the patterning layer to exposeportions of the first surface.
 17. The method of claim 15, whereinmetallurgically bonding the second surface to the copper nanorodbranches comprises heating the nanorod branches and the second surfaceto a temperature less than 400° C.
 18. The method of claim 15, furthercomprising underfilling the remaining space between the first surfaceand the second surface with a matrix material.
 19. The method of claim15, further comprising forming a nickel layer on the copper nanorods andcopper nanorod branches to prevent oxidation.
 20. The method of claim15, further comprising introducing a fluxing agent to remove oxide fromthe copper nanorods and copper nanorod branches.
 21. The method of claim15, wherein the copper nanorods are grown by one of sputter depositionand vapor deposition.